1. Field of the Invention
The present invention relates generally to an apparatus and method for detecting a neutral steering angle of a steering wheel system for a vehicle, the neutral steering angle thereof being defined as a steering angle at which the vehicle goes straight, and particularly relates to the apparatus and method for detecting the neutral steering angle for a short period of time immediately after the vehicle has traveled.
2. Background of the Art
A detection of a neutral steering angle is indispensable for a control of a four wheel steering carried out during a turn of the running vehicle.
A Japanese Patent Application First Publication Showa 62-201306 published on Sept. 5, 1987 exemplifies a previously proposed neutral steering angle detecting apparatus.
The previously proposed neutral steering angle detecting system includes a microcomputer for receiving a first pulse signal outputted by a steering angle sensor according to a steering angle of a steering wheel and a second pulse signal outputted by a vehicle speed sensor according to a vehicle speed. The microcomputer derives a relative steering angle to a steering position of the steering wheel on the basis of the first output signal when an ignition switch of the vehicle is turned on and derives a running distance of the vehicle from the second output signal of the vehicle speed sensor. The microcomputer derives an average value and variation width of the steering wheel sampled at a regular interval during the run of the vehicle whenever the vehicle has run at a predetermined reference distance. In a case where the variation width falls within a predetermined reference value, the microcomputer determines that the average value derived together with the variation width is the neutral steering angle. According to the previously proposed apparatus, since the variation width of the steering wheel is narrow and the microcomputer determines only the average value of the steering angle in a case where a possibility of running on a straight line is extremely high as the neutral steering angle, the neutral steering angle can always accurately be detected in cases where the turn in the same direction is repeated or where the steering angle is largely deviated from the neutral angle when the ignition switch is turned on.
However, the above-described neutral steering angle detecting apparatus cannot determine the average value of the steering wheel as the neutral steering angle when the vehicle does not run straight more than the predetermined distance after the ignition switch is turned on. Therefore, when the ignition switch is turned off and the neutral steering angle detected heretofore disappears, the neutral steering angle is newly updated whenever the ignition switch is turned on. Therefore, a detection accuracy cannot be improved by an accumulation of the detected data.